For underground construction and mining projects, boring machines are used to tunnel holes and displace soil below the earth's surface. It is essential for workers on the surface above the boring site to have knowledge of the bore machine's locations and its depth below the surface. Devices which approximate the surface location and depth of a boring machine are available in the prior art.
Prior art location devices, for example, the device of U.S. Pat. No. 4,806,869, include an assembly comprised of an oscillating magnetic dipole transmitter, a receiver and a display indicator. The magnetic dipole is affixed to the boring machine and is used to generate an electromagnetic field. A portable unit containing the receiver and signal processor is utilized above the boring site on the earth's surface. The receiver includes two or more antennas which are physically spaced at a predefined distance apart. Each antenna measures the relative electromagnetic field strengths at its particular position and generates an electrical signal in response thereto. The amplitude of each signal is proportional to the measured electromagnetic field strength and the signal's frequency is equal to that of the magnetic dipole.
To locate the boring machine's position, the signal processor relies on the principle that the electromagnetic field is the strongest at the surface point directly above the bore machine, because at this point, the signal processing device is the closest to the magnetic dipole. During operation, the operator is required to scan the receiver across the earth's surface area in the vicinity above the bore machine for the purpose of pinpointing the site where the magnetic field is the strongest. Once located, the processing device uses the strength of signals to calculate the bore machine's depth.
The algorithm the processor uses to calculate the bore machine's depth is well known in the art. Assuming the antennas generate signals S1 and S2 respectively, each signal is determined by the following equation: EQU S1=k/d.sup.3, and [1] EQU S2=k/(d+L).sup.3, [2]
where k is a constant dependent on the strength of the magnetic oscillator, and L equals the distance between the axes of the antenna.
Solving for the depth (d) of the dipole magnet on the bore machine, the processor uses the following equation: EQU d=L/{(S1/S2).LAMBDA.(1/3)-1}. [3]
Many of the prior art locating device assemblies operate at a frequency in the range of 30,000 hertz to 100,000 hertz. Signals at frequencies in this range tend to have satisfactory signal-to-noise ratio for accurate processing as contrasted with lower frequency signals.
The use of high frequency magnetic fields, however, have significant undesirable side effects. It is known that too high frequency electromagnetic fields create eddy currents within the metals. The eddy currents in the conductive metals are responsible for generating electromagnetic energy, which can significantly disrupt the magnetic field created by the dipole at the surface. Accordingly, if any metal is present at the boring site, the prior art location devices cannot differentiate between the dipole's electromagnetic field and the electromagnetic field originating from the surrounding metals. As a result, the ability of prior art location devices to pinpoint and calculate the boring devices position and depth is severely impeded.
To combat the problem of stray electromagnetic energy due to the presence of metals at the boring site, more recent location devices have relied on lower frequency dipole oscillators. This, however, leads to its own host of problems. Lower dipole magnet frequencies tend to create a lower signal to noise ratio at the antennas.
A number of noise reduction techniques have been employed in the prior art to eliminate some of the low frequency noise. One noise reduction technique employed in U.S. Pat. No. 4,806,869 is to employ for each antenna signal (S) a synchronous detector and a programmable gain amplifier (PGA). The synchronous detector modulates the signal (S) with a reference signal to reduce the signal (S) to a very small bandwidth. Since the noise present on the signal (S) is proportional to the square root of the system's bandwidth, noise reduction is achieved. In addition, the PGA amplifies the signal (S) to fulfill the dynamic range requirement of the system.
A number of problems are associated with the noise removal technique described above. The use of a synchronous detector and a PGA for each signal results in unavoidable thermal drifts, gain errors, modulation errors and significant measurement flaws between any two signals (S1 and S2 for example). The locating device's ability to accurately pinpoint the boring device's location beneath the surface and to calculate its depth is significantly impeded when inaccurate values between S1 and S2 are obtained with respect to one another.